1. Field of the Invention
The present invention relates to a differential coding apparatus adaptable for the differential coding of a fast video signal such as a TV signal.
2. Related Art of the Invention
Recently, with the development of picture display techniques, it has become necessary to provide a differential coding apparatus for carrying out the differential coding of a fast video signal such as a TV signal to be transmitted. A differential coding apparatus employing an intraframe.interframe differential coding is known and used as the differential coding apparatus for the TV signal. This differential coding apparatus has an intraframe prediction circuit and an interframe prediction circuit, and adaptively selects one of these circuits in response to the movement of a picture to be coded, so that a signal output by the selected circuit can be used as an optimum predicted value.
The intraframe prediction circuit uses, for example, a previous value prediction in which a predicted value of a pixel to be coded is produced on the basis of a reference pixel just prior to the pixel to be coded on the same horizontal scanning line. In this case, a time needed for producing the predicted value of the pixel to be coded on the basis of data of the reference pixel after such data was obtained becomes very short, and accordingly, the determination of the optimum predicted value must be carried out at a high speed. Therefore, in general, the logical elements used in the apparatus must be able to operate at a high speed.
However, such logical elements for high speed operation are expensive and consume much electric power, and accordingly, must have a large size as a heat countermeasure. Further, the high speed processing type logical elements have less durability against noise than low speed processing type logical elements.
Accordingly, one object of the present invention is to provide a differential coding apparatus in which the determination of the optimum predicted value can be carried out at a high speed without using a high speed processing type logical element.